Fluidic catalytic cracking continues to be the largest catalytic process in the world, and planning of FCC feed stock allocations continues to be a very complex problem, which must be addressed by petroleum refiners. For example, feeds of high economic opportunity are often heavy oils that require specialized FCC processing including a particular set of operating conditions that will realize a profitable product slate. Understanding the feed chemistry of petroleum crude oils and refinery streams has been a very important research topic for many years. Since FCC processes involve manipulation of carbon and hydrogen bonds, an accurate understanding of the feed composition and chemistry would allow the refiner to control operations involving catalytic and non-catalytic reactions. Ideally, the refiner would divide the feed into individual molecular components, however, petroleum FCC feeds are far too complex, such that the amount of analytic effort would be prohibitive.
Many methods have been suggested in the literature for characterization of petroleum feed stocks. Some researchers combine bulk analytical tests into correlating parameters. For example, the Viscosity Gravity Constant (J. B. Hill and H. B. Coats, Ind. Eng. Chem., 20, 641, 1928) is one such correlating parameter. As the name implies, the parameter uses specific gravity and Saybolt viscosity to characterize the oil. Another early parameter is the Watson K factor (K. M. Watson and E. F. Nelson, Ind. Eng. Chem., 25, 880, 1933), which is the cube root of the mean average boiling point divided by the specific gravity. For a given carbon number, the boiling point and specific gravity increase going from paraffins to naphthenes to aromatics, however, specific gravity increases more rapidly, such that high Watson K factors (greater than 12) correspond to an oil with high paraffinic content and low Watson K factor (less than 12) corresponds to an oil with higher aromatic content.
Riazi and Daubert (M. R. Riazi and T. E. Daubert, Ind. Eng. Chem. Proc. Des., Dev., 19, 289, 1980) showed that the Watson K factor is inadequate for the complete differentiation of molecular types, and developed a method to predict molecular composition of petroleum fractions using refractive index, Saybolt viscosity, and specific gravity. This method, which characterizes petroleum oils by molecular types rather than carbon content, is the standard given in the API Technical Data Book "Petroleum Refining," Chapter 2. B.4, (Report No. API-1-80, Apr. 18, 1980). The addition of refractive index complements the other types of tests and helps to better differentiate aromatic, paraffinic, and naphthinic compounds.
For a process such as catalytic cracking, which involves the breaking of carbon-carbon and carbon-hydrogen bonds, characterization of oils by carbon and hydrogen content is more useful than by molecular types. Two common methods used for characterizing the oil in this manner are the n-d-M method (ASTM D3238-80) and a method known as the Total method published by H. Dhulesia (Oil and Gas Journal, 51-54, Jan. 13, 1986). Both of these methods use refractive index as a key correlating property. The data used to develop the n-d-M method were obtained from fractions of five crudes, boiling between 480 and 890.degree. F. (see "Aspects of the Constitution of Mineral Oils," K. van Nes and H. A. van Westen, Elsevier Publishing Co., 1951). The Total method used thirty-three different FCC feed stocks which included some residual oil blends.
Although the above described methods have the advantage of characterizing FCC feeds by carbon and hydrogen content, they experience the objectional feature of being applicable for material boiling at temperatures less than 1000.degree. F.
Accordingly, it is an object of this invention to rapidly characterize potential heavy FCC feed stocks by carbon and hydrogen content.
It is a more specific object of this invention to analyze FCC feed quality for use in a model for computer simulation of an FCC reaction that predicts product yields.
It is a still more specific object to analyze FCC feeds in a simple and efficient manner, which can be routinely carried out in a refinery laboratory.
Still another objective of this invention is to develop a robust feed chemical analysis which is not dependent on feed source or pretreatment.